Fiber cement is a well-known material employed in many building components, such as siding, roofing and interior structures, and in pipes, particularly for waste water transport. Fiber cement typically comprises a mixture of cement (i.e., lime, silica and alumina), clay, a thickener, inorganic fillers such as calcium carbonate, and one or more fibrous materials. In the past, asbestos was commonly included as the fibrous material (see U.S. Pat. No. 4,216,043 to Gazzard et al.); because of the well-documented problems asbestos presents, now fiber cement typically includes a natural or synthetic fiber, such as acrylic, aramid, polyvinyl alcohol, polypropylene, cellulose or cotton. Fiber cement is popular for the aforementioned applications because of its combination of strength, rigidity, impact resistance, hydrolytic stability, and low thermal expansion/contraction coefficient.
To be used in siding or roofing components, fiber cement is often formed in sheets or tubes that can be used “as is” or later cut or otherwise fashioned into a desired shape. One technique of forming fiber cement articles is known as the Hatschek process. A fiber cement forming apparatus using the Hatschek process typically includes a porous fabric belt positioned on a series of support rolls. An aqueous fiber cement slurry of the components described above is created and deposited as a thin sheet or web on the porous fabric belt. The slurry is conveyed by the fabric belt over and through a series of rollers to flatten and shape the slurry. As the slurry is conveyed, moisture contained therein drains through openings in the fabric. Moisture removal is typically augmented by the application of vacuum to the slurry through the fabric (usually via a suction box located beneath the porous fabric). After passing through a set of press rolls, the fiber cement web can be dried and cut into individual sheets, collected on a collection cylinder for subsequent unrolling and cutting into individual sheets, or collected as a series of overlying layers on a collecting cylinder that ultimately forms a fiber cement tube.
The porous fabric used to support the slurry as moisture is removed is typically woven from very coarse (between about 2500 and 3000 dtex) polyamide yarns. Most commonly, the yarns are woven in a “plain weave” pattern, although other patterns, such as twills and satins, have also been used. Once they are woven, the yarns are covered on the “sheet side” of the fabric (i.e., the side of the fabric that contacts the fiber cement slurry) with a batt layer; on some occasions, the “machine side” of the fabric (i.e., the side of the fabric that does not contact the slurry directly) is also covered with a batt layer. The batt layer assists in the retrieval, or “pick-up,” of the slurry from a vat or other container for processing. Because of the presence of the batt layer(s), the fabric is typically referred to as a fiber cement “felt.”
Fiber cement felts typically include one or more base fabric layers that are formed into endless belts. The base fabric layers can be “flat-woven” and permanently joined after weaving into an endless belt, or the fabric layers can be woven in endless form. The longitudinal ends of flat-woven fabrics are generally joined in order to form an endless belt.
Some fiber cement products benefit from indentations in the surface, known as “marking,” in order to improve bonding of overlying layers. Such fiber cement product is typically used in roofing sheets, corrugated sheets, pipe and the like. Felts that produce marking often employ heavy spunbond CMD yarns in combination with spunbond-multifilament hybrid
MD yarns. However, these felts can suffer from contamination and reduced ability to mark over time. It may be desirable to provide a fiber cement product that addresses these shortcomings.